The present invention belongs to the technical field of liquid droplet ejection apparatus utilized for inkjet type recording method and the like and, more particularly, relates to a liquid droplet ejection apparatus of a new structure that does not have any nozzle for ejecting liquid droplets and also to an inkjet recording head utilizing this liquid droplet ejection apparatus.
Thermal inkjet type in which a portion of ink is rapidly vaporized by heating it by the use of a heater, so that, by the expansion force thereof and the like, ink droplets are ejected from nozzles, is utilized in various printers (See JP 48-9622 A, JP 54-51837 A, and the like).
Further, there are also known an electrostatic type and piezoelectric type inkjet printers constituted in such a manner that, by an actuator utilizing static electricity, a piezoelectric element or the like, a diaphragm is vibrated, so that, by the energy thereof, ink droplets, are ejected from nozzles (See JP 11-309850, etc.).
Recently, it is demanded to perform the image recording by such inkjet type more speedily. As for the method of enhancing the speed of image recording by inkjet type, it is important to make improvements in respect of the inkjet recording head such as improvement in the ejection frequency and increase-in the number of nozzles and, at the same time, to shorten the fixing time (drying speed).
Further, it is known that, for shortening the fixing time by raising the fixing speed, the method of reducing the amount of each ink droplet (reduction in size of liquid droplets) and increasing the number of target-hitting liquid droplets per picture element (the smallest unit for expressing images) is effective.
In the inkjet type, the size of the ink droplet is, basically, determined depending on the distance from the ejection unit to the nozzle and the size of the nozzle; and, for reducing the size of the ink droplet extending from the ejection unit to the tip end of the nozzle, the method of reducing the size (the diameter and length) of the nozzle is effective.
However, in case the nozzle diameter is less than 15 μm, the nozzle tends to get markedly choked up with the ink, as a result of which a stable operation can no longer be obtained. Further, nozzles are normally bored in a plate called an orifice plate (nozzle plate); and the thinning of this orifice plate is also effective for the reduction in size of the ink droplets. However, if the orifice plate is thinned, the orifice plate cannot support itself and thus hangs down, which results in the occurrence of inconveniences such as an insufficient feed of ink caused by the blockade of the ink feed paths due to this orifice plate and the shortage of the ejection pressure due to the fact that the orifice plate is so overwhelmed by the pressure at the time of ejection that it swells. Thus, the orifice plate is required to be thinned under the condition that it secures a sufficient rigidity; at present, it is difficult to reduce the thickness of the orifice plate to less than 10 μm.
In order to give a solution to such a problem, attempts are being made to reduce the size of ink droplets by the use of an inkjet recording head (hereinafter referred to as recording head) that has no nozzle (nozzle-less).
Known as an example of such nozzle-less recording heads is the so-called ultrasonic wave type recording head that is disclosed in JP 8-290587 A, JP 11-286104 A, and the like).
This recording head is constituted in such a manner that wavelets (capillary waves) are generated on the ejection surfaces (ink surfaces) of the ink droplets by acoustic waves, and the wavelength thereof is utilized as a substantial nozzle diameter, whereby about 10 nL (liters) to 1 pL of ink droplets are ejected. However, in the case of this recording head, the provision of actuators for producing acoustic waves, propagation structures for concentrating the acoustic waves on the ejection surfaces, and the like are necessary; therefore, each ejection mechanism is large, and the power consumption is also large, so that there is a fear that, for example, the realization of a high structural integration for enhancing the recording density may be difficult, though it is possible to render the ink droplets into a minute size.
As another method, there is known a recording head utilizing a minute structure as disclosed in JP 2001-88334 A. This recording head is constituted in such a manner that, in the vicinity of the ejection surface, a minute irregular structure for substantially holding the ink is formed, and this minute structure is provided with the functions to maintain the liquid surface, to cause meniscus growth, to perform ink severance, etc., whereby the ejection of minute ink droplets of a size less than several pL is realized. Even in the case of this recording head, however, there is the fear that the ink nozzles are choked up and the machining is difficult, though it is possible to render the ink droplets into a minute size.